(1) Field of the Invention
The present invention relates to a solid-state imaging device, and in particular relates to an improvement for resolving application unevenness in forming reflection prevention coating on a microlens, and for improving a yield ratio.
(2) Related Art
A solid-state imaging device formed by providing a semiconductor substrate with a plurality of light receiving devices (e.g. photodiodes) has such excellent characteristics as being small, light, having long life, reduced afterimage, and low power consumption. Use of such solid-state imaging devices is becoming more and more frequent these days as light receiving devices for video cameras and digital still cameras.
As disclosed in Japanese Patent Application Publication No. 2005-223130 and Japanese Patent Application Publication No. 2002-176156 for example, one solid-state imaging device results after forming a plurality of solid-state imaging devices on one semiconductor substrate (i.e. wafer), and then dicing the solid-state imaging devices into respective chips. The size of each solid-state imaging device is becoming more and more minute. So as to perform light collection for the purpose of obtaining sufficiently high sensitivity, it is designed to provide each one of light receiving devices on a wafer with one microlens in the production process.
A microlens is formed widely by a transparent resin such as an acrylic resin and polystyrene. However these materials have a reflection rate of about 10%. So as to restrain such reflection, reflection prevention coating has been conventionally applied onto microlenses made of such materials.
The reflection prevention coating is for example formed by the LB (Langmuir-Blodgett) method as is explained in FIGS. 14-16 (see Japanese Patent Application Publication No. H4-275459 for detailed explanation). In FIGS. 14-16, a water tank 101 is prepared in which water 102 is contained. At the surface of the water 102 in the water tank 101, a monomolecular layer film 107 composed of a hydrophobic group 1071 and a hydrophilic group 1072 is provided by being pressured from the side by means of pully blocks 104-106 for the purpose of maintaining the solid state of the monomolecular layer film 107. Then the substrate P is immersed and emersed several times in to and from the water 102, there by depositing the monomolecular layer film 107 several times onto the substrate P, to complete the reflection prevention coating (composed of layers of M1, M2 and M3). Other methods such as a droplet surface casting method and a spincoating method may also be employed. The spincoating method is a method by which a reflection prevention coating material is dropped onto a wafer, and the wafer is rotated at a predetermined rotation speed so as to spread the reflection prevention coating material thin.
However, if the spincoating method is adopted to form reflection prevention coating, application unevenness tends to occur comparatively frequently. This is because in the spincoating method, a centrifugal force incurred at the spincoating basically acts along the surface of the wafer, and so it is difficult to realize uniform application of the reflection prevention coating through out the minute microlenses that protrude from the wafer surface.
To be more specific, a semiconductor substrate inherently has minute bumps corresponding to the form of the microlenses. Therefore the coating material applied throughout the wafer in a rapid speed by a spincoating method cannot reach the top of each lens. As a result, uneven coating thickness and application non-uniformness are occasionally caused for the microlenses to lead to generation of image noises. These problems are described in Japanese Patent Application Publication H4-275459, for example, and having been considered problems to be solved conventionally.
So as to counter these problems, Japanese Patent Application Publication No. H5-55371 (see FIG. 13) creates a rectangular dummy pattern at each intersection of dicing lines. The technology attempts to alleviate the problems of application unevenness by regulating the flow speed of the applied material onto the wafer surface during spincoating by using the dummy patterns as barrier walls (i.e. step alleviating structure) However the mentioned technology originally attempts to alleviate the application performance of the material directed to the wafer surface itself, and so is insufficient in terms of a means to alleviate application unevenness and non-uniform film coating with respect to the microlenses that protrude from the surface of the wafer.
In this way, in the current state of the art, there is still room left for improvement in the process of applying a coating material onto microlenses.